A piezoelectric thin film resonator, which is one of acoustic wave devices, has been used in a filter and a duplexer of wireless devices including mobile phones. The piezoelectric thin film resonator has a structure designed to have a lower electrode and an upper electrode facing each other across a piezoelectric film.
The piezoelectric thin film resonator collaterally generates waves propagating in the planar direction (the lateral direction) called the lateral mode in addition to the thickness extension mode that vibrates in the film thickness direction (the longitudinal direction) of the piezoelectric film. The wave propagating in the lateral direction is reflected by the edge portion of a resonance region. This causes spurious to occur in resonance characteristics. When the piezoelectric thin film resonator in which spurious has occurred is used in a filter, a large loss called a ripple occurs in the passband. Thus, there has been suggested various methods of reducing spurious as disclosed in, for example, Japanese Patent Application Publication Nos. 2003-505906, 2007-6501, and 2005-159402 and International Publication No. WO2006/129532. It has also been known that waves propagating through the piezoelectric film have many modes as disclosed in, for example, Ken L. Telschow, and another, “Determination of Lateral Mode Dispersion from Full-field Imaging of Film Bulk Acoustic Resonator Motion”, IEEE ULTRASONICS SYMPOSIUM, 2003, p. 280-283.
However, the conventional methods of reducing spurious deteriorate a Q-value or an electromechanical coupling coefficient.